Digital processing of signals in telecommunications applications typically involves analyzing a telephone call to determine its status at various points in time. For example, it is well known to those of ordinary skill in the art that one can determine the status of a telephone call by analyzing call progress milestones such as, for example, busy, call-pickup, operator-intercept and so forth. Further, it is also well known to those of ordinary skill in the art that, under certain conditions, specific single-frequency tones, denoted as "call-progress" tones, are transmitted as analog signals over the telecommunications network to indicate call status. Examples of such "call-progress" tones include, without limitation, SIT tones (system intercept tones), answering machine tones, and so forth. As a result, there is a well recognized need in the telecommunications industry for apparatus which can detect and measure specific analog single-frequency signals, i e., "call-progress" tones, as a means for analyzing and monitoring call status.
In addition to the above, it is well known to those of ordinary skill in the art that processing and storing an analog signal by means of a computerized system typically requires some form of analog-to-digital conversion of the analog signal to a digital representation. Further, one well known method for performing an analog-to-digital conversion uses a linear digital encoding scheme. Although a linear digital encoding scheme provides a digital representation which has a great deal of fidelity, the linearly encoded digital signal often requires an unacceptably large amount of resources for storage. One method for reducing the amount of resources required to store a digital representation of the analog signal which is well known to those of ordinary skill in the art is to use a differential digital encoding scheme to provide the digital signal instead of the linear digital encoding scheme. Differential digital encoding schemes are well known to those of ordinary skill in the art collectively as delta modulation and there are many different forms thereof, such as, for example, DM (delta modulation), ADPCM (adaptive differential pulse code modulation), CVSD (continuously variable slope delta modulation), and so forth.
Because of the need to process analog signals by means of a computerized system, a typical apparatus for use in detecting and monitoring call progress tones in the prior art operates as follows. It detects an analog signal and converts it to digital form: (1) first, by using a differential digital encoding scheme to provide a digital signal which is suitable for efficient storage and (2) second, by using a linear digital encoding scheme to provide a digital signal which is used to detect and measure single-frequency tones. A typical apparatus in the prior art detects and measures single-frequency tones in the linearly encoded digital signal by methods such as, for example, the well known zero-crossing frequency detection method.
As one can readily appreciate from the above, the dual analog-to-digital conversion of analog signals which is typical of prior art apparatus, i.e., a differential digital encoded conversion of the analog signal and a linear digital encoded conversion of the analog signal, is inefficient and results in the need for extra hardware and extra software to achieve the dual conversion. Consequently, there exists a need in the art for apparatus and method for detecting and measuring single-frequency analog signals without the need to perform dual analog-to-digital conversions of the analog signals.
In addition to the above-identified need, there also exists a need in the art for apparatus and method for detecting and measuring single-frequency signals which occur in digitally encoded signals, whether the digital signals be digitally encoded by a differential encoding scheme or a non-differential encoding scheme such as a linear encoding scheme, a u-law encoding scheme, an A-law encoding scheme, a pulse code modulation (PCM) encoding scheme or so forth.